The impact of singlet oxygen on lipid oxidation in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 59

3.2.3 Reaction of singlet oxygen with lipids
Oxidation plays a fundamental role in the reduction of the quality of fats and oils
and therefore this matrix is also very useful to further discuss oxidation by
singlet oxygen versus oxidation by triplet oxygen. A significant amount of
research data is available on the impact of the so-called autoxidation in which
triplet oxygen is involved. Initially a lipid radical is formed in this oxidation
process, which requires a significant amount of energy. Therefore, autoxidation
is strongly accelerated by increased temperatures. Once the lipid radical is
formed it will react with other radicals such as triplet oxygen. In polyunsaturated
lipids with several double bonds present, the radical can be stabilized via
different resonance forms (Frankel, 2005). The observation that polyunsaturated
lipids have a higher relative oxidation rate confirms the role of radical formation
in the reaction mechanism of autoxidation (Table 3.2).
For singlet oxidation the reaction does not proceed via a radical type reaction.
The singlet state oxygen reacts directly with the unsaturated fatty acid via a
concerted `ene' addition mechanism (Fig. 3.1). For this reaction mechanism the
effect of the level of unsaturation of the lipids on the oxidation rate is only
limited because oxidation by singlet oxygen will not proceed via a radical
mechanism. The influence of temperature on the reaction rate is also negligible

Table 3.2 Relative oxidation rates of singlet and triplet
oxygen with unsaturated lipids

Type of oxygen Degree of unsaturation

C18:1 C18:2 C18:3

Triplet 1 27 77

310 4 410 4 710 4 Singlet

Fig. 3.1 Difference between lipid oxidation mechanism singlet and triplet oxygen.